1. Biological molecules: Carbohydrates 1 - Simple sugars

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  • Carbohydrates 1: Simple sugars
    • Carbohydrates in living organisms
      • Carbohydrates make up about 10% of the organic matter of a cell
      • The functions of carbohydrates include: energy source - released from glucose during respiration, energy store e.g. starch and structure e.g. cellulose
        • Some carbohydrates also form part of larger molecules e.g. nucleic acids, glycolipids
          • Carbohydratescontain the elements carbon, hydrogen and oxygen. The term carbohydrate essentially means 'hydrated carbon', because the elements are found in the proportions Cn(H2O)n
      • For every carbon present in a carbohydrate, the equivalent of a water molecule is also present
    • Simple sugars
      • The simplest carbohydratesare called the monosaccharides.
        • These are the monomers (basic units) of carbohydrates, all larger carbohydratesare made by joining monosaccharides together
      • Monosaccharides containing  3-6 carbons have very similar properties: soluble in water, sweet tasting and forms crystals
      • Monosaccharides are grouped according to their number of carbon atoms, 3-carbon: triose sugars, 5-carbon: pentose sugars, 6-carbon: hexose sugars
      • Most common: hexoses, these include glucose & fructose. One H2O for every carbon so written as C6H12O6, pentoses and hexoses tend to occur in nature as ring structures
    • Two forms of glucose
      • Glucose can be drawn as a chain or ring structure, the ring structure  can be shown 2 different ways: the H & OH at cabon1 may be above or below the plane of the ring
      • In alpha-glucose the OH at C1 is below the plane; in beta-glucose the OH at C1 is above the plane
      • The different structures lead to different chemical properties when joined to form polysaccharides
    • Joining monosaccharides and splitting disaccharides
      • Two monosaccharide molecules can be joined in a condensation reaction, forming a disaccharide
      • A new covalent glycosidic bond forms and water is eliminated
      • The reverse hydrolysis reaction uses a water molecule to break the glycosidic bond
      • e.g. sucrose = alpha glucose + fructose
      • Building the polysaccharides starch, glycogen and cellulose and breaking down larger molecules involves the making and breaking of glycosidic bonds


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